Cost-effective recycling of materials, such as glass, has become an increasingly important issue because of stresses on the environment and scarcity of resources. Concern about these issues has prompted governmental involvement, including the establishment of governmental guidelines, e.g., “bottle bills.” Because of governmental requirements and environmental awareness, American consumers are now recycling at a greater rate than ever before.
Increased recycling of materials reduces the amount of materials, such as glass, plastics, paper, etc. that enters land fills or other waste-disposal points. Additionally, recycling significantly reduces the need for manufacturers to use “virgin” materials, and thus preserves environmental resources. Further, the use of recyclables in place of virgin raw materials often reduces energy requirements, eliminates process steps, and reduces waste streams, including air emissions during product manufacturing. For example, recycled glass requires less energy and emits fewer contaminants during the glass manufacturing process than virgin raw materials do. Many states have placed demands on glass manufacturers that require new glass bottles to contain a minimum percentage of recycled glass. For example, in Oregon, glass container manufacturers are required to use at least 35% post-consumer cullet, which is broken pieces of glass. California has even more aggressive laws requiring glass manufacturers to increase use of recycled content to 65% by 2005.
However, there are associated issues of complying with these governmental mandates. Prohibitive sorting costs have made it difficult for suppliers to process an adequate quantity of single-colored recycled glass. Often, the glass coming into a material recovery facility (MRF) and/or a glass processing facility, e.g., a site where cullet is cleaned and prepared for shipment to glass manufacturers, is broken, contaminated with other materials, and of mixed color. Such material was previously unusable for glass manufacturing and was used in low value applications or simply placed in a landfill. Currently, however, regulatory requirements and other issues are forcing glass manufacturers to increase the amount of mixed cullet used in their furnaces which, in turn, has established the need to use mixed cullet in the manufacture of glass.
Generally, fewer problems exist when recycling clean sources of glass cullet that are of the same composition as products being manufactured. However, since most cullet is derived from consumer waste, the main issue is how well-segregated the different glass waste streams are, and the consequent level of contamination. Foreign material such as ceramics (such as pottery and china, from restaurants and/or bars); stones, gravel and/or dirt (from poor storage of cullet); ferrous metals (from bottle tops and other scrap); non-ferrous metals (lead foils from wine bottles or aluminum bottle caps); and organics (labels and excessive food residue) can all render a batch of cullet unusable.
In the glass-making process, non-ferrous metals can give rise to higher emissions, or can settle out as a layer at the base of the glass furnace. Ceramics may not be incorporated into the melt, and although organics will burn off in most furnaces, they create offensive odor and may present a problem in the crushing machinery used to produce the cullet. Thus, cullet quality and the levels of contamination are of major concern. It would be beneficial to develop a process for reusing mixed colored glass, wherein mixed cullet is used like color-sorted cullet, to make new and useful glass products.
Glass manufacturers are especially concerned about the quality of glass supplied from post-consumer streams. With glass batch formulations now able to contain increased amounts of mixed cullet (e.g., glass of mixed colors, typically green, amber, and flint/clear), the mixed cullet or single-colored glass received by glass manufacturers should consistently meet the specifications given by the suppliers. Glass manufacturers rely upon these specifications, which specify glass compositions and levels of impurities, to develop a glass batch recipe or formulation for manufacturing bottles.
Generally, the glass making operation is a continuous batch process where the cullet is melted and subsequently dissolved with other raw materials necessary to achieve the required final glass composition. The addition of colored cullet affects, amongst other processing variables, the absorptivity of the mixture and the rate at which it melts. Thus, each batch may require a different formulation of raw materials to achieve the desired final glass composition and consistent end product quality. Unfortunately, the supplied specifications may not be accurate, and glass manufactures often have to “guesstimate” batch formulations. For example, “pure” amber cullet from a processor may contain five or ten percent of other colors and impurity specifications for ceramic and organic contamination may be vague approximations. Any discrepancy between the specifications listed and the actual composition of the supplied glass results in glass that is not uniform in color compared to other glasses manufactured from different batches. This practice has been tolerated by the glass industry as long as cullet use levels were low. As cullet use levels increase, there is a need to reduce or eliminate variability, including both intra-batch and batch-to-batch variability, to ensure that the color of glass manufactured from the formulations is uniform. Thus, real-time analysis of mixed-cullet composition as it is introduced into the glass manufacturing process would be beneficial in ensuring uniformity of the as-manufactured glass bottles.
One method of using mixed colored cullet for glass manufacturing is described in U.S. Pat. No. 5,718,737, entitled, “Method of recycling mixed colored cullet into amber, green, or flint glass.” The '737 patent, herein incorporated by reference, describes how mixed colored cullet glass is recycled into amber colored glass by regulating the additive amounts of amounts of iron, carbon, sulfur, and sulfur compounds in the mixture to impart the desired reddish-brown hue. While the '737 patent provides a suitable method of using mixed colored cullet for glass manufacturing, it does not address the variability within the batch formulations as a result of the variations in the cullet supply. Nor does the '737 patent provide real-time analysis of the composition of mixed cullet as it is introduced into the glass manufacturing process in order to ensure end-product uniformity.
U.S. Pat. No. 6,230,521, entitled, “Method of recycling batches of mixed color cullet into amber, green, or flint glass with selected properties,” herein incorporated by reference, describes an automated method for recycling mixed colored cullet glass into new glass products. A computer-controlled process identifies the virgin glass raw materials, the desired target glass properties, the composition of a batch of mixed colored cullet, and the quantity of cullet to be used in the glass melt. The computer controlled process automatically determines the proper amounts of raw materials to add to the batch of mixed colored cullet so that recycled glass is produced that has the desired coloring oxides, redox agents, and glass structural oxides in the proper proportion. While the '521 patent provides a suitable method of recycling mixed color cullet glass into new glass products, it does not provide a way to remove impurities from the cullet within the glass manufacturing process prior to being introduced within the glass batch. One method of identifying cullet composition is provided by U.S. patent application Ser. No. 10/988,018, entitled “Method of Analyzing Mixed-Color Cullet to Facilitate Its Use in Glass Manufacture,” herein incorporated by reference.
Mixed cullet is processed and cleaned by entities, such as an MRF and/or a cullet processor, before it is shipped to a glass manufacturer. However, the supply of mixed cullet still often contains residual contaminants, such as ceramics and other impurities, that have escaped the cleaning process. The color composition of mixed color cullet depends on municipal sources and will vary over some time period and may or may not be reflected in supplier specifications when the glass is shipped to the glass manufacturer. These impurities within the mixed cullet are a serious concern to glass manufacturers and contribute to inconsistent quality and color of manufactured glass if adjustments to the batch formulation are not made.
There exists a need in the art for a method of ameliorating compositional variations within glass batch formulations. In order to further optimize the cullet and glass batch quality, there also exists a need in the art for a method and apparatus for further removing impurities from the cullet within the glass manufacturing process immediately prior to the cullet being introduced within the glass batch melt.